This first postcard seem to be from 1975. We moved to Luleå from Gävle in 1974. The number 75 on the postalstamp could refere to the cost also. But judging from the quality and spelling most likely four or five years old.
This yellow letter to my dad is most likely written in 1975 too, same style as the postcard.
This letter is clearly dated 1977 and the handwriting has improved a lot and the spelling too.